Modern cellular communication systems employ RF power amplifiers in their base stations, in order to provide communication means to subscribers. To achieve maximum utilization of available spectrum power amplifiers are required to amplify multiple Radio Frequency (RF) carriers. In addition to multiple RF carriers, each RF carrier employs a digital coding scheme such as Code Division Multiple Access (CDMA), which allows for multiple users to utilize the same spectrum. In addition to CDMA, there are systems that employ a modulation format known as Orthogonal Frequency Division Multiplexing (OFDM), in which the signal from a single user is first subdivided. Each subdivision is then modulated by a multiplicity of staggered sub carriers. The modulated sub carriers are then added up, thus causing large peak excursions in the RF signal carriers.
RF carriers modulated with large peak-to-average ratio signals require conventional RF amplifiers that are costly and relatively inefficient. One reason for such inefficiency is that a conventional RF power amplifier becomes efficient only during the occurrence of high output signal levels, i.e., when the instantaneous power output is large. However, during most of the time, the average power output is only a small fraction of the peak power, resulting in low overall efficiency, typically below 10%. Therefore, it is highly desirable to employ RF power amplifier circuits that incorporate high efficiency techniques, which can provide efficient operation over a wide dynamic back-off range.
One possible solution for improvement of efficiency in high power amplifiers involves the use of envelope elimination and restoration (EER). EER is a technique that employs high efficiency power amplifiers, which can be combined to produce a high efficiency linear amplifier system. In this method, a modulated input signal is split into two paths: an amplitude path through which the envelope of the modulated input signal is processed, and a phase path through which the phase modulated carrier of the modulated input signal is processed. In order for the EER technique to be effective the envelope of the modulated input signal is amplified with a highly efficient, narrow band amplifier. Conjunctionally, a high efficiency amplifier is used to amplify the high frequency phase modulated carrier with the amplified envelope signal. The EER technique is unique in that the amplifier, which generates the amplified envelope signal, also acts as the DC power supply to the high frequency amplifier. The efficiency of such EER amplifier systems can be calculated by multiplying the efficiencies of the two amplifiers. For example, if the efficiency of the first amplifier is 50 percent and that of the second amplifier efficiency is 40 percent, the total efficiency of the EER amplifier system will be 0.50*0.40=0.2 or 20 percent.
While these prior EER approaches may offer viable efficiency enhancement solutions, they add additional complexity due to support circuits and diminished linearity over operating conditions. Therefore, the desired combination of linearity and efficient operation, especially where large peak signals are present, has not yet been achieved.
Past amplifier measurements have confirmed that adequate inter-modulation distortion (IMD) performance requires that the amplifier's saturated power (Psat) must be greater than or equal to the amplified input signal's peak power (Ppeak). If a power amplifier has been tuned for a maximum peak power (Psat), this parameter basically depends on DC supply voltage (Vds). Due to their statistical nature, digital signal's peak power levels occur infrequently and their duration is very short near Psat levels. Therefore, it can be stipulated that the output stage of the power amplifier requires high drain supply voltage levels Vds only during these high power level excursions and for a short duration. Statistical signal analyses of digital signals indicate that the output stage of the power amplifier doesn't require high voltage during the majority of its operating time. It is advantageous to decrease Vds while tracking the input signal's envelope, which in turn will decrease average power consumption. A decrease in DC power consumption directly improves overall amplifier efficiency. On the other hand, any variation in Vds results in a gain and phase variation in a transistor stage. Gain and phase variation vs. Vds must be controlled if a dynamic Vds system is to be implemented. Constant AM-AM and AM-PM are the key requirements for power amplifier linearity.
Therefore, a need presently exists for a high efficiency power amplifier design which provides the desired linearity despite high peak to average power ratio input RF signals.